fix(protocol): align terminal receipt conformance

tools/native_conformance.rs

@@ -143,10 +143,12 @@ fn classify_gateway_feedback(encoded: &str) -> &'static str {
             1 => "invalid:length",
             2 if valid_fec_status(body) => "valid",
             2 => "invalid:field",
+            3 if body.is_empty() => "valid",
+            3 => "invalid:length",
             _ => "invalid:type",
         };
     }
-    if kind == 1 || kind == 2 {
+    if kind == 1 || kind == 2 || kind == 3 {
         return "invalid:direction";
     }
     match kind {